Blender internal: How to get a plain rendering of an object by ignoring its materials?
To add to the redundancy of answers in the comments:



You don't need to remove the materials for the objects in the scene or alter them in any way. Just select a material you want for the render and use it on the material section for the render layer. All of the other materials assigned for the object will be ignored.

From the Blender manual:

Material Override


  Overrides all material settings to use the Material chosen here.
  
  Examples of where this might be used:
  
  To check lighting by using a plain diffuse material on all objects.
  Render a wireframe of the scene.
  Create a custom render pass such as an anti-aliased matte or global coordinates.


EDIT:

Newer versions of blender don't use layers, but "collections" so the override function is in the scene>View Layer section.



Additional info:

Where is the Layers panel in Blender 2.8?

2.8 - Is it possible to assign (override) a material to a whole collection?